Rechargeable battery packs are used in portable devices. Cordless electric tools, for example, run on rechargeable battery packs that contain several lithium ion battery cells. Lithium ion is often the chemistry of choice for rechargeable battery packs because of its high capacity-to-size ratio and low self-discharge characteristics. The present invention will be described with reference to a battery charger system for charging or recharging a lithium ion battery pack, it being understood the present invention should not be limited thereto.
Lithium ion battery cells are charged or recharged in a process (hereinafter referred to as the “charging process”) that consists of three consecutive stages: a pre-charge stage, a quick charge-constant current (QC/constant-current) stage, and a quick charge-constant voltage (QC/constant-voltage) stage. During the first or pre-charge stage, a battery charger system charges a cell with a constant, low charge direct current (DC) current. When the voltage across the cell reaches a threshold (e.g., 2.8V), the pre-charge stage should stop, and the QC/constant-current stage should start. During QC/constant-current stage the battery charger system charges the cell with a constant, high charge DC current until the cell voltage reaches another threshold (e.g. 4.1V). When this threshold is reached, the QC/constant-current stage should end, and the QC/constant-voltage stage should start. During the QC/constant-voltage stage the battery charger system charges the cell at a fixed voltage (e.g., 4.1V). While the cell is charged during the last stage, the charging current should continuously fall. When the charging current falls below a threshold, the charging process should stop. Battery charger systems often include a microcontroller unit (MCU) that implements the charging process described above in accordance with an embedded program.